1. Field of the Invention
The present invention relates to a mobile communication system, a mobile terminal, and a mobile communication method.
2. Related Background Art
In the present specification, the term “transmission” is defined as generating and transmitting a packet to an apparatus itself, and the term “distribution” is defined as transmitting (and copying, if necessary) a packet generated by an another apparatus to a slave apparatus.
In a network (IP network) utilizing an IP (Internet Protocol) in a broad sense such as ICMP (Internet Control Message Protocol) and IGMP (Internet Group Management Protocol), a transmitter can perform multicast communication (IP multicast) for transmitting data of files and moving pictures to a plurality of receivers. A procedure for managing a multicast group utilizing IGMP is defined in the IFTF (Internet Engineering Task Force) RFC (Request for Comments) 1112.
In such multicast communication, a receiver is connected through a multicast router to a transmitter which transmits multicast data. In this case, the receiver and the multicast router are connected on a wired or wireless connection. Further, the receiver transmits an IGMP-join packet (path establishing request) or IGMP-Report packet (path establishing request) (hereinafter simply referred to as “a join packet”) to the multicast router. Upon receipt of the join packet (path establishing request), the multicast router recognizes that there is a need for distributing a multicast packet to the receiver and distributes a multicast packet supplied by the transmitter to the receiver.
The establishment of a multicast packet's distribution path in an IP network proceeds from a receiver to a transmitter. Specifically, when a receiver transmits a join packet (path establishing request), the multicast router that received the join packet (path establishing request) adds the multicast group that was requested by the join packet (path establishing request) to a distribution table of the router itself, and then transmits a multicast distribution path establishing request upstream towards a transmitter's end when viewed from the receiver. When an upstream multicast router receives the multicast distribution path establishing request, the router similarly updates its distribution table and transmits the multicast distribution path establishing request toward a further upstream multicast router.
When the multicast distribution path establishing a request reaches a multicast router that is connected to the transmitter, a multicast distribution path from the receiver to the transmitter is established. Let us assume that when a certain receiver transmits a join packet to a multicast group, another receiver has already received a multicast packet, and a multicast distribution path has thus already been established by a multicast router that is located on the multicast distribution path between the first receiver and the transmitter. Then, the multicast distribution path from the transmitter to the first receiver is established when the multicast distribution path establishing the request reaches that multicast router.
When a multicast distribution path is established, the transmitter transmits a multicast packet to the multicast router to which the transmitter itself is connected. In the case that the multicast router has a plurality of receivers in its coverage (located downstream of the same on an assumption that the transmitter is upstream of the same) and needs a plurality of multicast distribution paths to distribute the multicast packet to the plurality of receivers, the router makes a copy of the received multicast packet and distributes it to each of the multicast distribution paths.
In this manner, a multicast router that is in the position of having a plurality of multicast distribution paths to cover, i.e., a position where the multicast distribution path branches out, makes copies of a multicast packet and transmits the packet to multicast distribution paths that are in need of distribution. This eliminates any unnecessary increase in traffic on multicast distribution paths to allow the band of the network to be efficiently utilized.
IP multicast as described above is achieved when a multicast router capable of IP layer processes copies and distributes a packet. In the case of a data link layer switch used in a LAN (Local Area Network) for connecting multicast routers and receivers, since it does not normally performs IP layer process, a multicast packet distributed by a multicast router is distributed to all paths whether any receiver exists in the paths or not, which results in an inefficient use of the band.
As a method for distributing a multicast packet to paths in which receivers exist for preventing such inefficient use of a band, studies are being conducted on a method called IGMP snooping in which a data link layer switch records the ports where the join packets transmitted by the receivers have arrived to recognize paths in which receivers exist; and, when a multicast packet is distributed from a multicast router, the switch distributes the multicast packet only to the paths in which receivers exist. For example, such a method is disclosed in Considerations for IGMP and MLD snooping switches, IETF Internet-draft by Morten Jagd Christensen (Thrane & Thrane) et al. (Non-Patent Document 1).
The use of the method makes it possible to distribute a multicast packet only to paths in which receivers exist even in a LAN and to thereby utilize the band efficiently.
When a receiver is connected to a repeater such as a data link layer switch on a wired basis, the receiver cannot receive a multicast packet unless the multicast packet is distributed to the path to which the receiver is connected. Therefore, the repeater is required to make and distribute copies of the multicast packet in the quantity of multicast distribution paths that have receivers downstream. In cases where the repeater is a radio base station through which a plurality of receivers receive multicast packets, the plurality of receivers can simultaneously receive an electric wave signal of a multicast packet distributed from the radio base station. Therefore, the radio base station is not required to make copies of the multicast packet in the quantity of the receivers, and it can distribute the packet to the plurality of receivers using the same band as that used for distributing a packet to a single receiver.
When a receiver receiving multicast packets is connected to a data link layer switch through a radio base station, the receiver may move to another radio base station. When the receiver has moved between the radio base stations, the path connecting the previous radio base station to the data link layer switch is different from the path connecting the current radio base station to the data link layer switch. Therefore, in order to receive a multicast packet through the current radio base station, a new multicast distribution path to the current radio base station must be established at the data link layer switch. However, as described above, a multicast distribution path is established when a receiver transmits a join packet, and a data link layer switch recognizes the path in which the receiver exists from the port where the join packet (path establishing request) arrives. Normally, a receiver transmits a join packet only when it joins a multicast group and when it responds to an IGMP query packet (hereinafter simply referred to as “query packet”) that it receives, an IGMP query being a request for a check of the presence of receivers which is issued by a multicast router to the receivers downstream to check whether there is another receiver belonging to an associated multicast group. Therefore, when a receiver has moved, the receiver cannot receive a multicast packet until a query packet is transmitted by a multicast router that is present upstream of the relevant data link layer switch, and the receiver transmits a join packet (path establishing request) in response to the query packet. Since a default value of the interval at which query packets are transmitted is 120 seconds, the receiver cannot receive a multicast packet for a period of about 60 seconds on average when there is no other receiver belonging to the same multicast group downstream of the radio base station into which the receiver has moved.
When there is no other receiver belonging to the same multicast group downstream of the radio base station that the receiver has left, the data link layer switch cannot recognize that the receiver does not exist any more on the paths to which the previous radio base station is connected until a query packet is transmitted by the multicast router, and no receiver responds by a join packet to the query packet through the path to which the previous radio base station is connected. Therefore, even after the receiver has moved out, multicast packets are distributed to the path to which the previous radio base station is connected in spite of the fact that there is no receiver to receive the multicast packets, the packets being also distributed on wireless basis. Thus, the band is similarly wastefully used for a period of about 60 seconds on average.
As a method for deleting a multicast distribution path quickly so as to prevent such wasteful use of a band, there is a method for IGMP in which a receiver transmits an IGMP leave packet (or a withdrawal request, which is hereinafter simply referred to as “leave packet”) for requesting withdrawal from a multicast group to a multicast router. Upon receipt of a leave packet (withdrawal request), a multicast router transmits a query packet to check whether there is any receiver and, if there is no receiver, it performs a process for deleting multicast distribution paths. However, since a receiver must receive multicast packets until a point in time immediately before it moves, it is very difficult for the receiver to transmit a leave packet (withdrawal request) immediately before it moves.
In this manner and in order to solve the problem that multicast packets are not distributed to a receiver from a radio base station into the coverage where the receiver has moved and that distribution of multicast packets to the paths connected to the radio base station from which the receiver has left cannot be stopped, a method is being studied in which an MSA (Mobility Support Agent) is provided at each radio base station to establish and delete multicast distribution paths using the MSA. Specifically, before a receiver moves, it transmits packets requesting establishment of a multicast distribution path at the destination to an MSA present on a path to which the radio base station at the destination, a sufficient time being accommodated for the transmission. After the receiver moves, lit transmits a packet requesting deletion of a multicast distribution path at the previous location to an MSA present on a path to which the radio base station at the previous location is connected. Upon receipt of such packets, the MSA, as an agent of the receiver, transmits a join packet (path establishing request) or a leave packet (withdrawal request) to a multicast router. Thus, a multicast distribution path is established at the destination before the receiver moves, and a multicast distribution path is deleted at the previous location after the receiver moves. For example, such a method is disclosed in Non-Patent Document 2 listed later.
Another method has been conceived in which a join packet (path-establishing request) and a leave packet (withdrawal request) are directly transmitted to multicast routers at the destination of a movement and the point of departure without providing MSAs. According to this method, the strength of the connection of radio links is used to predict the destination, and a join packet (path-establishing request) is transmitted to a multicast router present on a path connected to the radio base station to which the movement is likely to occur. After the movement, a leave packet (withdrawal request) is transmitted to a multicast router present on a path connected to the radio base station at the previous location. Thus, multicast distribution paths are established and deleted without providing MSAs. For example, such a method is disclosed on Patent Document 1 and Non-Patent Document 3.
According to another method currently under study and unlike the methods disclosed in the two Non-Patent Documents in which a multicast distribution path is established before a receiver moves, a receiver that has moved detects the movement and transmits a join packet (path establishing request) to establish a multicast distribution path before a query packet is transmitted by a multicast router. According to this method, when a receiver receives an advertisement packet transmitted by a multicast router after it has moved and the multicast router which has provided the advertisement is different from multicast routers at the previous location of the receiver, the receiver detects that it has moved and transmits a join packet (path establishing request). Thus, the time required to receive a multicast packet after the movement is reduced. For example, such a method is disclosed in Non-Patent Document 4.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-258826
Non-Patent Document 1: Considerations for IGMP and MLD snooping switches, IETF Internet-draft, draft-ietf-magma-snoop-05.txt, January 2003 by Morten Jagd Christensen (Thrane & Thrane) et al.
Non-Patent Document 2: AN IP Mobility Support Architecture for the 4GW Wireless Infrastructure, Proceedings of the 1999 Personal Computing and Communication Workshop (PCC '99) by Jiang Wu (Department of Teleinformatics, Royal Institute of Technology, Stockholm)
Non-Patent Document 3: Study on Method of IP Multicast Reception in Mobile Network, The Institute of Electronics, Information and Communication Engineers (IEICE), Technical Research Report No. NS2001-245, IN2001-201 by Yuki Moritani et al.
Non-Patent Document 4: Study on Multicast Communication on Mobile IP Network, Information Processing Society of Japan (IPSJ), Report of Research on Mobile Computing and Wireless Communication No. 16-6 by Takashi Hashimoto et al.